This circuit is a test circuit for 555 Timer pin 5 modulation input here I use a transistor current as a constant current sink the transistors base input greater then 0.8 divided by the emitter resistor is the sink current this allows you to input a linear input to the base and frequency should be a function off base input still testing this circuit finding that you get linear sweep off the frequency from 1 volt to 5 as you see in the plot the input the blue trace is the ramp input to the base and yellow trace is the voltage at pin 5 which drops from its initial voltage 6 volts to 3.8 where it it levels off for the maximum input the input resistance seen by the capacitor is HFE * the emitter resistor
